1. Field of the Invention
The present invention relates to a belt for a continuously variable transmission in which a number of metal blocks are supported by an aggregate of metal rings formed by laminating a plurality of pieces of endless metal rings one on another.
2. Description of the Related Art
A belt for a continuously variable transmission has been known in Japanese Patent Publication (Kokoku) No. 2-22254 according to which, in order to enhance the durability of the belt, the belt comprises an aggregate of metal rings and metal blocks, with mesh-like protuberances formed on the inner peripheral surfaces of the metal rings forming the aggregate of metal rings, thereby decreasing the coefficient of friction. A belt has also been known in Japanese Utility Model Publication (Kokai) No. 62-97344 according to which oil grooves are formed in the saddle surfaces of the metal blocks which come into contact with the inner peripheral surface of a metal ring of the innermost position of the aggregate of metal rings, thereby holding an oil film and decreasing the coefficient of friction.
In the conventional metal rings forming the aggregate of metal rings, the innermost metal ring that comes into contact with the saddle surfaces of the metal blocks and other metal rings (at the second position and outer positions) are all formed into the same. However, while the metal ring of the innermost position comes into contact with the saddle surfaces of the metal blocks, the inner peripheral surfaces of the other metal rings come in contact with the outer peripheral surfaces of their adjacent metal rings, and hence their contacting portions have different coefficients of friction. It has been determined by measurement that the coefficient of friction on the inner peripheral surface of the metal ring of the innermost position that comes into contact with the saddle surfaces of the metal blocks, is generally about twice as great as the coefficient of friction on the inner peripheral surface of the other metal rings.
As will be described later in detail, when a metal belt wrapped round a drive pulley and a driven pulley circulates, the tension acting on the aggregate of metal rings of the metal belt undergoes a periodic change. When the coefficient of friction is the same throughout the inner peripheral surfaces of all metal rings, the tension acting on the metal rings changes by the same amount. In practice, however, the coefficient of friction on the inner peripheral surface of the metal ring of the innermost position is greater than the coefficient of friction on the inner peripheral surfaces of the other metal rings and hence, the tension acting on the metal ring of the innermost position changes by an amount greater than the amount of change of the tension that acts on the other metal rings. As a result, the metal ring of the innermost position receives a stress greater than that exerted on the other metal rings, and loses durability. The durability of the metal ring of the innermost position imposes a limitation on the durability of the whole metal belt.